Dielectric cavity optical resonators have attracted increasing attention in sensing applications, including biosensing. Typically these resonators consist of either microspheres, or of planar-waveguide-based disk or ring cavities. The size of these types of resonators typically ranges from approximately 20 microns to a few millimeters for microspheres and from 5 microns to several hundreds of microns for ring- or disk-shaped resonators. Such small spheres and ring- or disk-shaped resonators are often referred to as microresonators.
In the most common configuration in microresonator-based sensors, a microresonator is placed in close proximity to an optical waveguide such as optical fiber whose geometry has been specifically tailored, for example, tapered or etched to a size of 1-5 microns. The tapering modifications to the waveguide result in there being a substantial optical field outside the waveguide, and thus light can couple into the microresonator and excite its eigenmodes. These eigenmodes may be of various types, depending upon the resonant cavity geometry. For spherical and disk cavities, the modes of interest for sensing applications are usually the so-called “whispering gallery modes” (WGMs), which are traveling waves confined close to the surface of the cavity. Since the WGMs are confined near the surface, they are well-suited to coupling with analytes on or near the sphere surface. FIG. 2 schematically illustrates the WGM 202 electric field distribution for light propagating within a planar disk microresonator cavity 210. The field intensity, E, is schematically illustrated in FIG. 2 for the WGM 202 along the cross-section line A-A′.
For ring cavities based on single-mode waveguides, the modes are those of the single-transverse-mode channel waveguide, under the constraint that the path traversed corresponds to an integral number of wavelengths. Other cavity geometries, such as Fabry-Perot resonators using single-mode waveguides with Bragg grating reflectors, or multimode rectangular cavities, have familiar standing-wave resonances as their eigenmodes.
When microresonators made with low loss materials and with high surface reflectivity and quality are used, the loss of light confined in the resonant modes is very low, and extremely high quality factors, also known as Q-factors, can be achieved, as high as 109. Due to the high Q-factor, the light can circulate inside the resonator for a very long time, thus leading to a very large field enhancement in the cavity mode, and a very long effective light propagation path. This makes such devices useful for non-linear optical and sensing applications. In sensing applications, the samples to be sensed are placed on or very near the resonator's surface, where they interact with the evanescent portion of the resonant electric field available outside the microresonator. Due to the enhanced field and the increased interaction length between the light and samples, the microresonator-based optical sensors feature high sensitivity and/or a low detection limit.
In the most-commonly-pursued configuration, in which a microsphere resonator is coupled to a tapered optical fiber, there are practical difficulties associated with realizing efficient and stable coupling. First, in order to make the optical field in the fiber core available outside the fiber's surface, the fiber must be tapered to a few microns in diameter. This commonly results in a relatively long (a few cm) and fragile tapered region. Second, the relative position of the microsphere and the fiber taper must be held constant to within a few nanometers if the optical coupling and the Q-factor are to remain constant. This is difficult with a free sphere and tapered fiber.
In another configuration commonly used to couple with microspheres, an angle polished fiber is put into contact with the microsphere. In this case the problems associated with the fragility of the fiber taper are overcome, but there are still significant difficulties in positioning the microsphere properly on the fiber tip. Furthermore, light which is not coupled into the microsphere, representing the so-called “through port” signal, is not confined to a fiber and is therefore difficult to collect and analyze.
There is a need for improved methods and structures for coupling a waveguide to a microresonator.